1. Field of the Invention
This invention relates to the use of SiC for IR radiation sensing and resistance control.
2. Description of the Related Art
IR radiation is presently sensed, for applications such as measuring the power or energy output from IR lasers, by pyroelectric, bolometer and calorimeter detectors. Generally, pyroelectric and bolometer detectors employ materials that directly absorb IR; pyroelectric materials include lithium niobate and tantalum niobate; bolometer materials include silicon, germanium, gallium arsenide, metal-oxide ceramic thermistors and various glasses. Generally, calorimeter detectors employ materials that must be coated with an IR-sensitive absorption coating. The performance of IR sensing detectors is constrained by the IR-sensor material's capacity to absorb energy without damage; which limits the maximum energy/power intensity, maximum exposure time and minimum volume and area for the sensor. All pyroelectric, bolometer and calorimeter materials have limited thermal shock tolerances.
Materials currently used for the direct absorption of IR radiation are easily damaged if they get too hot (above about 400.degree. C.), or if the sensor temperature increases too rapidly. To sense the power or energy output of medium and high power IR lasers, present materials are exposed to only a fraction of the output IR energy by interposing a beam splitter or a disbursing medium between the laser and the sensor. However, this results in reducing the power or energy measurement to an estimate. Furthermore, presently available sensors for medium and high power IR sources require fan or water cooling, and are subject to calibration drift.
A related application for IR radiation sensitive materials is in sensing the temperature of other materials that are heated by IR radiation. For example, the rapid thermal annealing (RTA) process, used extensively in the semiconductor industry, uses high intensity IR lamps to ramp the temperature of semiconductor wafers (principally silicon) by several hundred degrees centigrade per second. Wafer temperatures are presently monitored by remote sensing using either optical or IR pyrometers, or by direct contact thermocouples.
Pyrometers measure the wafer temperature by absorbing radiation emitted from the wafer surface through a transparent view port in the RTA process chamber wall. This type of temperature sensing is limited by a need to know the precise emissivity of the observed wafer surface, a need to prevent particulates or dispersive gas between the wafer surface and view port or any deposits on the view port or wafer surface, and a requirement to avoid any changes in the wafer surface such as contamination or chemical reaction or texture changes.
Thermocouples measure the wafer temperature by touching its surface. Key problems with this approach are that the thermocouple must be enclosed to prevent reactions between it and the wafer, the thermocouple-wafer contact is very difficult to ensure because the wafer is spun at about 1200 rpm to ensure uniform processing, and contacting the wafer with the thermocouple actually changes the local temperature.
Materials presently used for IR radiation power or energy sensing could theoretically also be used as temperature sensors, but they would not survive the environments or temperatures often required, particularly RTA processing in which temperatures can reach 1300.degree. C.